When using wood for industrial purposes, it is important to exercise careful quality control to ensure that the material properties are properly matched to the desired end uses. However, wood is a natural material, and can have a wide range of mechanical properties, even within a single species. Consequently, it is important to be able to sort the wood into different grades, each with specific material properties. Such sorting allows the wood resource to be used efficiently and economically.
Typical wood properties of interest include specific gravity, moisture content, grain direction, stiffness and strength. These properties are of industrial importance both individually and in combination. Perhaps the most challenging property to estimate is wood strength. This is because it is controlled in a subtle way by several different wood characteristics. Accurate identification of wood strength is essential when producing lumber that is to be used for structural applications.
The traditional method for estimating wood strength is by visual observation. The process involves human observation of wood features such a knots and grain distortions. Wood strength is then estimated from the observed features using standardized empirical rules. The visual method is subject to several uncertainties and has only modest strength estimation capabilities.
The bending method is the most common mechanical process for estimating wood strength. The procedure involves bending the wood and measuring the force required to produce a given deflection. The bending method gives a better estimate of wood strength than visual grading, but the estimation accuracy is still only moderate. The available accuracy is mainly limited by the coarse resolution of the stiffness measurement. This measurement is typically done over a 4 foot span, while the main strength controlling features, the knots, are only 0.5-2 inches in diameter. Additionally, a bending machine cannot measure the first and last 2 feet of a board. Bending machines also require intensive maintenance.
X-ray absorption provides a more accurate method of wood strength grading. Schajer describes the method in U.S. Pat. No. 4,941,357 entitled “Method and Apparatus for Estimating the Strength of Wood.” The procedure uses X-ray absorption to indicate the gross density of the wood. The method has fine resolution, comparable to, or finer than, the size of the knots. The X-ray measurements extend from end to end of each board, and so all the material is examined. In addition, the measurements are non-contact, thereby minimizing machine maintenance needs.
Recent advances in computing power have enabled more sophisticated mathematical techniques to be used for wood sorting applications. These mathematical techniques can take into account multiple factors that control wood strength and other properties. They combine the effects of these factors to achieve more accurate wood property estimates. For maximum effectiveness, the mathematical techniques need to work with large amounts of measured data. These data should preferably include measurements of several independent wood properties and they should have fine spatial resolution. The X-ray method provides measurements that partially meet this need. They have fine resolution, but however, they indicate only one wood property, bulk density.
The invention described here is a device that is capable of simultaneously providing fine-resolution of up to five independent wood dielectric properties. These dielectric properties can be used to indicate wood mechanical properties. The invention provides the large amount of fine-resolution, multi-property data that are needed to achieve superior wood strength estimates using the sophisticated mathematical techniques. The same measurements and mathematical techniques can be used to estimate other useful wood properties such as moisture content and stiffness.